Add screenshots to the debug dashboard documentation

Include screenshots of the traces list page and trace detail page in
the Dashboard pages section so that readers can see what the UI looks
like before setting it up.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: dahlia

docs/manual/debug.md

@@ -288,6 +288,8 @@ trace, it displays:
  -  **Activity count**
  -  **Timestamp**
 
+![Traces list page of the debug dashboard](debug/traces-list.png)
+
 The page automatically polls the JSON API every 3 seconds and refreshes when
 new traces are detected.
 
@@ -317,6 +319,8 @@ records captured during the trace.  Each log entry displays:
  -  **Logger category** (e.g., `fedify.federation.http`)
  -  **Message** with expandable properties
 
+![Trace detail page of the debug dashboard](debug/trace-detail.png)
+
 ### JSON API
 
 A JSON API endpoint is available at `/__debug__/api/traces` which returns